1. Field of the Invention
The present invention relates to a carrier detector for detecting the coming of the burst and detecting a carrier, and a frequency controller using the same in a satellite communication and mobile communication system.
2. Description of the Related Art
In SCPC (Single Channel per Carrier) systems using voice activation, for example, a demodulator should perform a burst operation. There has been proposed a method for improving the burst operation of a demodulator by recognizing the coming of the burst through use of a carrier detector. FIG. 45 is a block diagram showing a conventional carrier detector described in "O db Eb/No Burst Mode SCPC Modem with High Coding Gain FEC" (IEEE, ICC '86 56.4). FIG. 46 is a flow diagram showing an operation of the detector shown in FIG. 45B. In FIG. 45B, the numerals designate respectively: 12, a voltage-controlled oscillator (hereinafter referred to as VCO); 13, a phase detector for comparing a phase of a received signal and a phase of an output of the VCO 12; 14, a PLL (Phase Locked Loop) circuit composed of an output from the VCO 12 and the phase detector 13; 15, a mixer for mixing the received signal and the output of the VCO 12; 16, an LPF for averaging noise in the output from the mixer 15; and 17, a square calculating section for calculating a square value of an output signal of the LPF 16.
The operation of this detector will now be described with reference to FIGS. 45A, 45B, 46 and 47. FIG. 47 shows an example of a format of a burst signal to be received. In FIG. 47, the numerals designate respectively: 20, a pattern for a carrier recoverying (normally `11` or `00` pattern, hereinafter referred to as CR pattern); 21, a pattern for bit timing recovering (normally `10` pattern, hereinafter referred to as BTR pattern); 22, a burst synchronization pattern called unique word (hereinafter referred to as UW pattern); 23, a data section for carrying data; 24, a non-signal section (noise only); 25, burst composed of the CR pattern 20, BTR pattern 21, UW pattern 22 and the data section 23. Since the carrier detector shown in FIG. 45 operates continuously, if the received signal is represented by s (t), this s (t) would receive the CR pattern, the BTR pattern, the UW pattern, the data section, and the non-signal section (noise only). But for simplification, in this case only the following three cases will be considered:
(a) CR pattern;
(b) Non-signal section (noise only); and
(c) Data section.
For each of these three cases, the operation of the carrier detector will now be described. In this case, the UW pattern and the data section are supposed to be pseudo noise pattern (hereinafter referred to as PN pattern).
(a) Carrier only:
If the received signal is represented by s (t), and the output of the VCO 12 with the PLL 14 being locked is represented by V (t), s (t) and V (t) will be expressed as the following equations 1 and 2: EQU s(t)=exp(j2.pi..DELTA.ft)+n(t) (1)
.DELTA. f:carrier frequency
n (t):noise component EQU V(t)=exp{j2.pi..DELTA.ft+j.theta.e+.theta.n(t)} (2)
.theta. e:steady state phase error
.theta. n (t):phase jitter caused by noise
The mixer 15 multiplies s (t) with V (t), the product of which is passed through the LPF 16. The square calculating section 17 calculates the square z of the output r of the LPF 16 to provide power P (z) of z.
(b) Non-signal section (noise only)
The received signal s(t) can be represented by the equation 3: EQU s(t)=n(t) (3)
With this s (t), the P (z) could be obtained in the same manner as in the case of (a).
(c) Data section:
The received signal s (t) is represented by the equation 4: EQU s(t)=exp{-j(2.pi..DELTA.ft+D)}+n(t) (4)
D:random variable representing phase modulation
With this s (t), the P (z) could be obtained in the same manner as in the case of (a).
The distribution of P (z) could be represented as in FIG. 45A, so it would be possible to detect the existence of the CR pattern by setting a suitable threshold from this FIG. 45. As a result, by comparison of the output P (z) of the square calculating section 17 with a threshold, it would be possible to detect the coming of the burst and to detect the carrier.